Oh, I know how frustrating that is! As the mother of a 2 1/2 year old who STILL doesn't sleep through the night, I sympathize with your situation. (My 10-week old, however, sleeps 8-9 hours at night already, so I am CONVINCED that kids are just naturally good sleepers or not.)
At 8 months, your granddaughter should be taking two naps per day, one in the morning and one in the afternoon. My daughter used to nap 1-2 hours in the morning and 2 hours in the afternoon.
I assume your granddaughter is sleeping at night? Perhaps before naps you could do an abbreviated version of her night time routine, like giving her a bottle, rocking, then reading a book and putting her down.
One thing that helped my non-sleeper sleep was a white noise machine, which I got at Bed, Bath, and Beyond for $10. She likes the waterfall sound best, which honestly sounds like static to me, but it does help her sleep. Plus it drowns out any other noises in the house and outside.
I do know, too, that when kids reach a new milestone (you mentioned she's just started crawling), their sleep suffers because they get so excited and want to practice their new skill. Plus teething is hard on sleep. I'd ask her pediatrician if you can give her some Motrin before naps/bed.
Good luck!